Constant Current LED Driver (Rev. C) - Educypedia

TPS61042
SLVS441C–DECEMBER 2002–REVISED MARCH 2007
APPLICATION INFORMATION
Inductor Selection, Maximum Load Current, and Switching Frequency
Inductor fall time:
ip L
t
fall Vout–Vin
For tfall ≥ 400 ns
ip Vin
I
load max 2Vout
ip2 L Vin
I
load max (VoutVin) (2 ip L2 400 ns Vin)
ip 500 mA Vin
η = expected converter efficiency. Typically between 70% to 85%
100 ns
L
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The PFM peak current control scheme of the TPS61042 is inherently stable. The inductor value does not affect
the stability of the regulator. The selection of the inductor together with the nominal LED current, input, and
output voltage of the application determines the switching frequency of the converter.
The first step is to calculate the maximum load current the converter can support using the selected inductor.
The inductor value has less effect on the maximum available load current and is only of secondary order. A
good inductor value to start with is 4.7 µH. Depending on the application, inductor values down to 1.0 µH can be
used. The maximum inductor value is determined by the maximum on time of the switch of 6 µs (typ). The peak
current limit of 500 mA (typ) must be reached within this 6 µs for proper operation. The maximum load current of
the converter is determined at the operation point where the converter starts to enter the continuous conduction
mode. The converter must always operate in discontinuous conduction mode to maintain regulation.
Depending on the time period of the inductor current fall time being larger or smaller compared to the minimum
off time of the converter (400ns typ), the maximum load current can be calculated.
for t fall ≤ 400 ns
with:
L = selected inductor value
(Peak inductor current as described in the peak current control section)
The above formula contains the expected converter efficiency that allows calculating the expected maximum
load current the converter can support. The efficiency can be taken out of the efficiency graphs shown in
Figure 2 and Figure 3 or 80% can be used as an accurate estimation.
fs(ILOAD)
2 I V V V
LOAD O I F
I
(LIM) Vin
If the converter can support the desired LED current, the next step is to calculate the converter switching
frequency at the operation point, which must be ≤1 MHz. Also the converter switching frequency should be much
higher than the applied PWM frequency at the CTRL pin to avoid non-linear brightness control. Assuming the
converter shows no double pulses or pulse bursts (Figure 13, Figure 14) on the switch node (SW) the switching
frequency at the operation point can be calculated as:
100 ns2 L
L
with:
I (LIM) = minimum switch current limit (500 mA typ)
L = selected inductor value
I (LOAD) = nominal load or LED current
V F = Rectifier diode forward voltage (typically 0.3 V)
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